In this work a new concept of the effective rigidity, $R_{\rm eff}$, of a neutron monitor (NM) for detection of a ground-level enhancement (GLE) is introduced so that the integral omnidirectional flux of solar energetic particles (SEP) with rigidity above it is proportional to the NM response to GLE. The effective rigidity depends on the location (the cutoff rigidity and altitude) of the NM but is independent on the SEP spectrum in a broad range of the spectral parameters. Use of this concept allows one to directly evaluate the SEP fluence. The exact value of $R_{\rm eff}$ depends on the NM yield function. We have tested four modern yield function: Mi13 (Mishev et al., JGR:SP, 2013), Ma16 (Mangeard et al., JGR:SP, 2016), CM12 (Caballero-Lopez & Moraal, JGR:SP, 2012) and CD00 (Clem & Dorman, SSR, 2000) and recommend Mi13 and CM12 for practical use. The method was applied to three recent GLEs: $\#$69, 70 and 71, for which a reconstruction of the energy spectra without any assumptions on the exact spectral shape was performed. We found that the SEP fluence is not well described by a power-law function but demonstrates a steepening at higher energies, which can be approximated by the modified power-law or Ellison-Ramaty spectral forms.